It appears that not everyone understands the concept that slower traffic on the highway should stay right, and the left-hand lane should be used for passing. According to state law:

On all roadways, a person driving a vehicle proceeding at less than the normal speed of traffic at the time and place and under the conditions then existing shall drive the vehicle in the right-hand lane then available for traffic or as close as practicable to the right-hand curb or edge of the roadway, except when overtaking and passing another vehicle proceeding in the same direction or when preparing for a left turn at an intersection or into a private road or driveway.

A rightfully angry member of the community on the Arizona Republic website posted a link to the state driver's manual, which includes the following explanation:

The right lane is used for entering and exiting, and for slow traffic. The left lane is used by higher-speed traffic.

This discussion about people clogging up left-hand lanes in Phoenix also plays out in a recent 117-comment Reddit thread, where it's made clear that left-lane cloggers are a common Phoenix frustration.

For good measure, we checked with the Arizona Department of Public Safety for input. Here's what spokesman Carrick Cook told us:

The only stance we would take is that people should adhere to the posted speed limit. We can understand the frustrations of drivers who drive behind someone going below the speed limit. Maintaining a safe traveling flow of traffic is the most advantageous for drivers. Also, there is no acceptable speed above the posted limit.

Just in case you didn't catch that the first time: "Maintaining a safe traveling flow of traffic is the most advantageous for drivers."